This invention relates to an image transfer device for an electrophotographic copier and the like for transferring toner from the surface of its photosensitive drum onto a sheet of transfer paper.
In an electrophotographic copier and the like, an electrostatic latent image is typically formed on the surface of a photosensitive drum and after electrically charged toner is electrostatically attached to this latent image, the resultant toner image is electrostatically transferred onto a transfer paper sheet by a corona ion current such that a toner image is finally formed on the front surface of this transfer sheet. A conventional image transfer device for carrying out the last of the steps above, that is, the step of transferring a toner image onto a transfer sheet, includes a charger box 2' disposed opposite to the photosensitive drum 1' as shown in FIG. 5 with a charger line 3' stretched inside the charger box 2'. A high voltage is adapted to be applied to this line 3' to produce a corona ion current. Numeral 4' indicates a paper guide for directing a transfer paper sheet 10' glidingly thereover onto the surface of the photosensitive drum 1' where the back surface of the transfer sheet 10' becomes charged by the corona ion current as explained above and toner is transferred from the drum surface to the front surface of the sheet 10'.
When an image transfer device of this type is used as explained above, the transfer of toner should take place ideally only after the sheet 10' comes into contact with the surface of the photosensitive drum 1' because this will make it certain that the toner image formed on the drum surface is accurately and reliably transferred onto the transfer paper sheet 10'. With a conventional image transfer device, however, the back surface of an incoming sheet 10' becomes charged as shown in FIG. 5 before the sheet 10' comes into contact with the surface of the drum 1'. A Coulomb force is thereby generated between such prematurely charged parts of the sheet 10' and toner particles on the surface of the drum 1', causing the toner particles to fly onto such parts of the sheet 10' and adversely affecting the quality of the image formed on the sheet.
If the incoming transfer sheet 10' is very thin, or tends to bend easily, the position at which it comes into contact with the surface of the drum 1' becomes unpredictable especially in cases where the separation is relatively large between the paper guide 4' and the surface of the drum 1'. In a situation like this, even the degree to which toner particles may be expected to scatter is unpredictable.